Broadband probes for impedance tuners

ABSTRACT

A multi-section probe and a tapered probe for impedance tuners to broaden the band width of the probes and hence the band width of the tuners. Each section of the multi-section probe has a nominal length equal to one quarter wavelength at a midpoint of the operating band. The tapered probe has a length equivalent to a plurality of one quarter wavelengths of the frequency midpoint. The multi-section probe and the tapered probe are configured to transform the characteristic impedance of the tuner transmission line step-by-step or continuously to a target impedance value.

BACKGROUND

Mechanical impedance tuners use probes to simulate impedance values forvarious microwave and RF measurements such as load pull or source pullmeasurements or noise parameter measurements. The transmission line ofthe tuner may be a slab line. A slab line is a type of transmission linehaving opposed parallel slabs (or plates) with a center conductorbetween them. The slabs act as the outer conductor of the transmissionline. The probes are movable in a direction transverse to the centerconductor of the transmission line of the tuner, and also in a directionalong the center conductor. As the probe moves closer to the centerconductor, the impedance mismatch increases, while the mismatchdecreases as the probe is moved away from the center conductor. Theprobes can generate high reflections and act to transform thecharacteristic impedance of the slab line to other impedance values. Amajor shortcoming is, as is known to microwave engineers, the narrowband of these probes.

Commonly owned U.S. Pat. No. 7,589,601 describes multi-section probes,in which the sections are separated by gaps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric cutaway view of an automated tuner with a movingcarriage and a probe.

FIG. 2 is a cutaway view of an impedance tuner with a multi-sectionprobe.

FIG. 3A is a cutaway side view of an impedance tuner with an alternateembodiment of a multi-section probe. FIG. 3B is a bottom view of theimpedance tuner and probes of FIG. 3A, showing that the trough of eachprobe section is different from the trough of the other probe sections.FIG. 3C is a diagrammatic end view, illustrating the probe sectiontrough widening and change in the cross-sectional profile of therespective probe sections.

FIG. 4 is a diagrammatic isometric view of an exemplary embodiment of atapered probe with a straight taper angle.

DETAILED DESCRIPTION

FIG. 1 schematically depicts an exemplary embodiment of an automated,slab line tuner system 10. In this embodiment, a base plate 12, an endplate 14 and planar conductor slabs 18, 20 are fabricated of a metal ormetalized dielectric material. A center conductor 16 is supportedbetween the slabs 18, 20, and by a coaxial connector (not visible inFIG. 1) fitted into the end wall 14. An electrically conductive probe 22is mounted on a carriage 24 for motion transverse to the centerconductor axis. A probe motor 26 drives the probe 22 along thetransverse path toward or away from the center conductor axis. Thecarriage is driven along a path parallel to the center conductor axis,by a leadscrew 30 driven by a carriage drive motor 28. In an exemplaryembodiment, moving the carriage primarily results in changing the phaseof the reflection, and moving the probe vertically (transversely)primarily changes the magnitude of the reflection; hence together(horizontal and vertical movements) change the impedance presented atthe reference plane (usually the end of a connector at the end of thecenter conductor) by the tuner. Exemplary slab line impedance tuners aredescribed, for example, in U.S. Pat. Nos. 7,589,601; 8,823,392; and8,907,750, the entire contents of which are incorporated herein by thisreference.

In microwave and RF circuits, a multi-section quarter wave transformerhas been used to generate wide band quarter wave transformers formicrostrip or coaxial lines. These multi-section transformers can bedesigned in most case either using the maximally flat filter response orChebyshev filter response. Quarter wave transformers are described, forexample, in “Microwave Engineering,” Second Edition, David M. Pozar,John Wiley & Sons, Inc., 1998, at Chapters 5.4 to 5.8, pages 271-295(hereinafter referred to as “Pozar”).

In accordance with an aspect of the invention, instead of using onesection of a probe in a mechanical impedance tuner to transform thecharacteristic impedance of the main line, to a very low or very highimpedance value in one step, the characteristic impedance is transformedstep-by-step or probe section by probe section using a plurality ofadjacent probe sections to intermediate impedance values to reachfinally the target impedance value. This method, if done properly,widens the bandwidth significantly, in fact arbitrarily depending on thenumber of sections; see, e.g. Pozar at pages 277-278. How to calculatehow many sections are needed for a desired bandwidth and the varyingimpedance values is explained in the literature, e.g. Pozar at page278-286.

In accordance with aspects of the invention, multi-section probes aredescribed for use in impedance tuners, wherein each probe sectioncorresponds to a particular impedance value for the transmission line.There are different methods of realizing such multi-section probes. Inone embodiment, the same probe section is used several times as neededfor the desired broad bandwidth, with the probe sections positioned nextto each other but at stepped heights relative to the center conductor ofthe slab line. In this example, the impedance value is at a given heightor vertical position of the probe in relation to the slab line centerconductor. An example is illustrated in FIG. 2, in which the probe 22includes three probe sections 22A, 22B and 22C, with each probe sectionhaving a length equal to one quarter wavelength at the center frequencyof the band. The probe sections are mounted together in a fixedrelationship, and fitted to a probe drive (not shown in FIG. 2) formovement in a ganged relationship toward or away from the centerconductor. The probe sections may be attached to a probe bracket 22D,for example, which is connected to the probe drive by a post structure22E. The probe sections could also be fabricated as an integralone-piece structure, instead of several separately constructed sectionsassembled together.

An exemplary design technique for designing a multi-section probe is asfollows:

1. Determine the desired specifications as to what maximum reflection,e.g., 0.9 reflection magnitude, is needed in which characteristicimpedance environment, e.g., 50 ohm, and the desired bandwidth, e.g.between 0.65 and 9 GHz, along with the desired reflection magnitudeΓ_(m) at the band edges, e.g., 0.82 reflection magnitude. This will alsofix the center frequency of the design and the length of each probesection as quarter wave length at the center frequency, e.g. 4.825 GHzin the above example.

2. For each type of design, Chebyshev or binomial (maximally flat),determine the design constants and number of sections needed:

-   -   Estimate initially the number of sections N. Then estimate the        constant

$A = {2^{- {({N + 1})}}{\ln( \frac{R_{L}}{Z_{0}} )}}$

-   -    where R_(L) is the low impedance target and Z0 is the        characteristic impedance of the system, usually 50 ohm. Then,        using the reflection Γ_(m) needed at the band edge, calculate        the bandwidth using

${\Delta\; f} = {{2\; f_{0}} - {\frac{4\; f_{0}}{\pi}{arc}\;{\cos\lbrack {\frac{1}{2}( \frac{\Gamma_{m}}{A} )^{\frac{1}{N}}} \rbrack}}}$

-   -    where f₀ is the center (design) frequency of the probe. If the        bandwidth is acceptable, continue, if it is too narrow or too        wide, re-estimate the number of sections and repeat iteratively        until a satisfactory bandwidth is obtained and the number of        sections N is then determined.

3. Once the number of sections N is determined, then determine therequired characteristic impedance value for each section, using thefollowing:

-   -   Estimate the marginal reflection coefficients

$\Gamma_{n} = {A\frac{N!}{{( {N - n} )!}{n!}}}$

-   -    and then determine the characteristic impedance of each section        iteratively by Z_(n+1)=Z_(n)e^(2Γ) ^(n) starting with the 50        ohm/Z0 load.

4. Estimate the height of each section above the center conductor tosimulate the calculated characteristic impedance for that section. Theheight of each section in this context is the distance or gap betweenthe center conductor and the top of the trough of the section. This stepcan typically be performed through the use of a full 3D electromagneticfield (EM) simulator such as HFSS (by ANSYS) or CST (marketed byComputer Simulation Technology), or any other full 3D electromagneticsimulator.

5. Once the height of each section is determined this way, in the finalstep, simulate the performance of the design of the whole probe usingthe 3D EM simulator to verify the broad band response and make the finaltuning adjustments to the heights if further adjustments in heights areneeded.

Another embodiment of a multi-section probe uses the same height for allsections but varies the cross-sectional profile (also known as thetrough) to make the trough wider and wider for successive probesections. The troughs are configured to allow the probe sections whenbrought closer to the center conductor to straddle the center conductor.In this case, the probe could be made as a single integrated probe, withthe trough made wider and wider every one-quarter wavelength. FIGS.3A-3C illustrate an exemplary multi-section probe 22′ using probesections 22A′, 22B′, 22C′, which are mounted together at equal heightsrelative to the center conductor 18, but with different troughconfigurations. As with the probe 22 of FIG. 2, the multiple probesections are mounted adjacent one another, and mounted for gangedmovement (or made as a one single unitary structure). FIGS. 3B and 3Cshow that the trough of probe section 22B′ is wider than the trough ofprobe section 22A′, and that the trough of probe section 22C′ is widerthan the trough of probe 22B′. Thus the probe sections have troughs eachof which is wider than the previous adjacent probe section, with thecharacteristic impedance increasing with each widening trough. Thetrough width corresponds to the impedance obtained using full 3D EMsimulation.

Note that the probe bracket and post for connecting to the probe driveare omitted from FIGS. 3A-3C for clarity.

A further embodiment of the probe is a combination of the first twoembodiments, i.e. a multi-section probe in which both the probe heightand the trough profile vary, i.e. from probe section to probe section.

An even better but more difficult to realize probe design is to use atapered quarter wave transformer instead of multi-section transformers.This tapered transformer has a continuous change of the impedance of thetransformer instead of stepped change by multi-section transformer.Various methods such as exponential taper, triangular taper,Klopfenstein taper are used for different taper shapes and differentapplications. It is known that the Klopfenstein taper is the optimaltaper shape for these types of quarter wave transformers. See, forexample, Pozar at chapter 5.8, pages 288-295.

FIG. 4 illustrates an exemplary embodiment of a tapered probe 22″ with atrough 25. The height of the probe is tapered, increasing from a firstend (left in FIG. 4) to the opposite end (right in FIG. 4). The probe22″ will be mounted to the probe drive by a post (not shown in FIG. 4)such that the top surface is parallel to the center conductor.

Both techniques described above, i.e., the stepped height or varyingtrough shape, could be also used for the tapered probe, whether it is atriangular or exponential or Klopfenstein taper.

An exemplary sequence of steps to design a tapered probe is as follows:

1. Establish the required specifications for center frequency,bandwidth, desired reflection at center frequency and desired reflectionat band edges

2. Estimate the needed length L of the tapered probe (the longer, thewider the band width). One can estimate this length from the multisection probe design (e.g., Chebyshev), so that the length of thetapered probe is equal to the sum of the lengths of the sections of themulti-section probe structure.

3. Determine the constants and the impedance profile for the probe from0 up to the length L. This profile depends on the chosen taper style,exponential, triangular or Klopfenstein.

4. Once the impedance profile is determined, now one needs to estimatethe probe profile. This is done by estimating the how much the probeheight above center conductor corresponds to that impedance. This may bedone using a 3D EM simulator such as HFSS or CST. The probe height forthe entire probe is estimated, i.e. how the taper profile or taperheight varies along the probe.

5. Simulate the probe performance using a 3D EM simulator. If necessary,the parameters might need to be adjusted or tuned such as probe lengthand probe height profile.

In a further embodiment, the tapered probe may incorporate both heighttapering and trough profile tapering, i.e. both the height and thetrough profile vary continuously along the probe length.

All these embodiments can be implemented with probes that touch the slablines or that do not touch the slab lines.

Although the foregoing has been a description and illustration ofspecific embodiments of the subject matter, various modifications andchanges thereto can be made by persons skilled in the art withoutdeparting from the scope and spirit of the invention.

The invention claimed is:
 1. A probe for a slab line impedance tunersystem operable over a frequency bandwidth, the tuner system includingopposed slab conductor planes and a center conductor disposed betweenthe slab conductor planes, a probe carriage and a drive system formoving the probe carriage in a longitudinal direction parallel to thecenter conductor, the probe comprising: a tapered electricallyconductive probe section having a nominal length dimension along thelongitudinal direction, wherein the tapered probe section has across-sectional profile defining a trough configured to straddle thecenter conductor as the probe is moved transversely toward the centerconductor; wherein the probe section is supported for movement along thecenter conductor and in a direction transverse to the center conductor;and wherein the nominal length dimension is sufficient to provide adesired characteristic impedance transformation for the frequencybandwidth, wherein the characteristic impedance of the tuner system istransformed continuously by the probe to intermediate impedance valuesto reach a target impedance value.
 2. The probe of claim 1, wherein thetapered probe has a height relative to the center conductor which variescontinuously along the probe length from an initial height closest tothe center conductor to a final height furthest away from the centerconductor.
 3. The probe of claim 1, wherein the cross-sectional profileof the tapered probe section is varied such that the trough becomeswider along the probe length from a first end of the probe to a secondprobe end.
 4. The probe of claim 1, wherein the tapered probe sectionhas a height relative to the center conductor which varies continuouslyalong the probe length from an initial height closest to the centerconductor to final height furthest away from the center conductor, andwherein the cross-sectional profile of the probe section is varied suchthat the trough becomes wider along the probe length from a first end ofthe probe to a second probe end, and wherein the height is a measure ofthe distance from a top of the trough to the center conductor.
 5. Theprobe of claim 1, wherein the nominal length dimension is equivalent toa plurality of one quarter wavelengths at a frequency at the midpoint ofthe frequency bandwidth.
 6. The probe of claim 1, wherein the taperedprobe section comprises one of a triangular taper shape, an exponentialtaper shape or a Klopfenstein taper shape.
 7. A probe for a slab lineimpedance tuner system operable over a frequency bandwidth, the tunersystem including opposed slab conductor planes and a center conductordisposed between the slab conductor planes, a probe carriage and a drivesystem for moving the probe carriage in a longitudinal directionparallel to the center conductor, the probe comprising: a tapered probesection having a nominal length dimension along the longitudinaldirection, wherein the taper is such that the characteristic impedanceof the probe varies smoothly along the longitudinal direction from afirst end of the probe section to a second end of the probe section;wherein the probe section is supported for movement along the centerconductor and in a direction transverse to the center conductor; andwherein the length dimension of the probe section is sufficient toprovide a desired characteristic impedance transformation for thefrequency bandwidth, wherein the characteristic impedance of the tunersystem is transformed continuously along the probe to intermediateimpedance values to reach a target impedance value.
 8. The probe ofclaim 7, wherein the tapered probe section has a height relative to thecenter conductor which varies continuously along the probe section in alongitudinal direction from an initial height closest to the centerconductor at the first end to a final height furthest away from thecenter conductor at the second end.
 9. The probe of claim 7, wherein thetapered probe section has a cross-sectional profile defining a troughconfigured to straddle the center conductor as the probe is movedtransversely toward the center conductor and wherein the cross-sectionalprofile varies such that the trough becomes continuously wider along theprobe length in a longitudinal direction from the first end of the probeto the second end of the probe.
 10. The probe of claim 7, wherein: thetapered probe section has a cross-sectional profile defining a troughconfigured to straddle the center conductor as the probe is movedtransversely toward the center conductor; and the tapered probe sectionhas a height relative to the center conductor which varies continuouslyalong the probe section in a longitudinal direction from the first endto the second end, from an initial height closest to the centerconductor to a final height furthest away from the center conductor, andwherein the cross-sectional profile of the tapered probe is varied suchthat the trough becomes continuously wider along the probe length in alongitudinal direction from the first end to the second end.
 11. Theprobe of claim 7, wherein the tapered probe section comprises one of atriangular taper shape, an exponential taper shape or a Klopfensteintaper shape.
 12. A slab line impedance tuner system operable over afrequency bandwidth, the tuner system comprising: a slab linetransmission including opposed slab conductor planes and a centerconductor disposed between the slab conductor planes; a probe; a probecarriage carrying the probe; a carriage drive system for moving theprobe carriage in a longitudinal direction parallel to the centerconductor; a probe drive system for moving the probe in a transversedirection relative to the center conductor to position the probe closerto or further away from the center conductor; and wherein the probeincludes: a tapered conductive probe section having a nominal lengthdimension along the longitudinal direction, wherein the tapered probehas a cross-sectional profile defining a trough configured to straddlethe center conductor as the probe is moved transversely toward thecenter conductor; wherein the probe section is supported for movementalong the center conductor and in a direction transverse to the centerconductor; and wherein the nominal length dimension is sufficient toprovide a desired characteristic impedance transformation for thefrequency bandwidth, wherein the characteristic impedance of the tunersystem is transformed continuously by the probe to intermediateimpedance values to reach a target impedance value.
 13. The tuner systemof claim 12, wherein the tapered probe section has a height relative tothe center conductor which varies continuously along the probe lengthfrom an initial height closest to the center conductor to final heightfurthest away from the center conductor.
 14. The probe of claim 12,wherein the cross-sectional profile of the tapered probe section isvaried such that the trough becomes wider along the probe length from afirst end of the probe to a second probe end.
 15. The probe of claim 12,wherein the tapered probe section has a height relative to the centerconductor which varies continuously along the probe length from aninitial height closest to the center conductor to final height furthestaway from the center conductor, and wherein the cross-sectional profileof the tapered probe section is varied such that the trough becomeswider along the probe length from a first end of the probe to a secondprobe end, and wherein the height is a measure of the distance from atop of the trough to the center conductor.
 16. The tuner system of claim12, wherein the nominal length dimension is equivalent to a plurality ofone quarter wavelengths at a frequency at the midpoint of the frequencybandwidth.
 17. The tuner system of claim 12, wherein the probe touchesat least one of the opposed slab conductor planes.
 18. The tuner systemof claim 12, wherein the probe does not touch the opposed slab conductorplanes.
 19. The tuner system of claim 12, wherein the tapered probesection comprises one of a triangular taper shape, an exponential tapershape or a Klopfenstein taper shape.
 20. A slab line impedance tunersystem operable over a frequency bandwidth, the tuner system comprising:a slab line transmission including opposed slab conductor planes and acenter conductor disposed between the slab conductor planes; a probe; aprobe carriage carrying the probe; a carriage drive system for movingthe probe carriage in a longitudinal direction parallel to the centerconductor; a probe drive system for moving the probe in a transversedirection relative to the center conductor to position the probe closerto or further away from the center conductor; and wherein the probeincludes: a tapered probe section having a nominal length dimensionalong the longitudinal direction, wherein the taper is such that thecharacteristic impedance of the probe varies smoothly along thelongitudinal direction from a first end of the probe section to a secondend of the probe section; wherein the probe section is supported formovement along the center conductor and in a direction transverse to thecenter conductor; and wherein the length dimension of the probe sectionis sufficient to provide a desired characteristic impedancetransformation for the frequency bandwidth, wherein the characteristicimpedance of the tuner system is transformed continuously along theprobe to intermediate impedance values to reach a target impedancevalue.
 21. The impedance tuner system of claim 20, wherein the taperedprobe section has a height relative to the center conductor which variescontinuously along the probe section in a longitudinal direction from aninitial height closest to the center conductor at the first end to afinal height furthest away from the center conductor at the second end.22. The impedance tuner system of claim 20, wherein the tapered probesection has a cross-sectional profile defining a trough configured tostraddle the center conductor as the probe is moved transversely towardthe center conductor and wherein the cross-sectional profile varies suchthat the trough becomes continuously wider along the probe length in alongitudinal direction from the first end of the probe to the second endof the probe.
 23. The impedance tuner system of claim 20, wherein: thetapered probe section has a cross-sectional profile defining a troughconfigured to straddle the center conductor as the probe is movedtransversely toward the center conductor; and the tapered probe sectionhas a height relative to the center conductor which varies continuouslyalong the probe section in a longitudinal direction from the first endto the second end, from an initial height closest to the centerconductor to a final height furthest away from the center conductor, andwherein the cross-sectional profile of the tapered probe is varied suchthat the trough becomes continuously wider along the probe length in alongitudinal direction from the first end to the second end.
 24. Theimpedance tuner system of claim 20, wherein the probe touches at leastone of the opposed slab conductor planes.
 25. The impedance tuner systemof claim 20, wherein the probe does not touch the opposed slab conductorplanes.
 26. The impedance tuner system of claim 20, wherein the taperedprobe section comprises one of a triangular taper shape, an exponentialtaper shape or a Klopfenstein taper shape.